“Aerodynamics plays a huge part with trunking trailers whereas weight becomes a more important consideration for secondary distribution rigid vehicles,” he insists.
Even at today’s low diesel prices, however, Owens believes that getting the choice and/olr compromise right can generate “significant savings in fuel costs” – while getting it wrong can “mean a worthless investment”.
For Owens, there are two distinct issues any fleet engineer should make consider.
Firstly, what payload do you expect to be carrying and secondly, what is the average speed of the vehicle?
“The results from this simple research will determine whether your vehicle will be most affected by either momentum or relative wind speed,” he explains.
That said, Owens warns operators to remember that low tare weight semi-trailers (say, below 6 tonnes) have little impact on the overall total laden weight, if the vehicle is running close to its maximum 44 tonnes.
“1 tonne tare [off trailer weight] may offer an opportunity to add more payload [which] won’t directly affect your overall weight and fuel consumption, but will reduce the number of trips to transport the same load,” he continues – indirectly saving “a high percentage” in fuel.
That argument changes radically, however, when it comes to a 500kg weight saving on a 7.5 tonne rigid.
“This relatively large weight reduction will have a far greater effect on fuel economy for a strictly urban vehicle than any aerodynamic widget you can stick on it,” he says.
What about aerodynamics? Owens suggests that at 56mph, ”roughly half” of fuel burned is directly attributable to aerodynamic drag – meaning that an aerodynamic with average fuel savings of just 5% will shave £1,466 off the annual diesel fuel bill.
“Under the same circumstances, a Teardrop trailer with in-operation average fuel saving of 11.3% will reduce fuel consumed by over £3,000 per annum,” he says (based on 80,000 miles per year and bulk diesel at 76.21ppl).
His conclusion: trunking trailers that spend much of their time running between 40 and 56mph “benefit hugely” from aerodynamic interventions, while urban trailers on stop-start cycles with multiple drop-offs are “unlikely to reach high enough speeds where aerodynamics will have any effect at all”.
Pictured: an Argos max-cube longer semi trailer with a tare weight of 14.9 tonnes, capable of carrying 60 UK pallets.